Constructible number
In geometry and algebra, a real number r {\displaystyle r} is constructible if and only if, given a line segment of unit length, a line segment of length | r | {\displaystyle |r|} can be constructed with compass and straightedge in a finite number of steps. Equivalently, r {\displaystyle r} is constructible if and only if there is a closed-form expression for r {\displaystyle r} using only integers and the operations for addition, subtraction, multiplication, division, and square roots.